linux-kernel - [PATCH v2] time: Avoid signed overflow in timekeeping_delta_to

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Message-Id: <1479331203-10349-1-git-send-email-cmetcalf@mellanox.com>
Date:   Wed, 16 Nov 2016 16:20:03 -0500
From:   Chris Metcalf <cmetcalf@...lanox.com>
To:     John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Laurent Vivier <lvivier@...hat.com>,
        David Gibson <david@...son.dropbear.id.au>,
        "Christopher S . Hall" <christopher.s.hall@...el.com>,
        linux-kernel@...r.kernel.org
Cc:     Chris Metcalf <cmetcalf@...lanox.com>
Subject: [PATCH v2] time: Avoid signed overflow in timekeeping_delta_to_ns()

This bug was originally fixed in commit 35a4933a8959 ("time:
Avoid signed overflow in timekeeping_get_ns()").  When the code was
refactored in commit 6bd58f09e1d8 ("time: Add cycles to nanoseconds
translation") the signed overflow fix was lost.  Re-introduce it
in a less subtle way by changing the type of "nsec" to unsigned
and adding a comment explaining why.

Signed-off-by: Chris Metcalf <cmetcalf@...lanox.com>
---
v2: just use "u64 nsec" to fix the signed shift problem

 kernel/time/timekeeping.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 37dec7e3db43..8c06a2aa9b5f 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -302,7 +302,7 @@ static inline u32 arch_gettimeoffset(void) { return 0; }
 static inline s64 timekeeping_delta_to_ns(struct tk_read_base *tkr,
 					  cycle_t delta)
 {
-	s64 nsec;
+	u64 nsec;   /* Avoid possibility of a negative right shift. */
 
 	nsec = delta * tkr->mult + tkr->xtime_nsec;
 	nsec >>= tkr->shift;
-- 
2.7.2